The present invention relates to a method for track counting which, in the course of track counting, takes account of the direction of movement of an objective lens of an apparatus for reading from and/or writing to an optical recording medium with regard to tracks situated on the said recording medium, and also to a corresponding apparatus for reading from and/or writing to an optical recording medium.
In conventional apparatuses for reading from and/or writing to optical recording media, for example DVD-RAMs, generally a track error signal is generated, which serves as a basis for the track regulation in the respective apparatus. One method for generating this track error signal is the DPP method, for example. The DPP method (“Differential Push-Pull”) is described e.g. in EP 0 745 982 A2.
In accordance with the DPP method, a laser beam is split into three beams, namely a primary beam and two secondary beams which scan adjacent tracks of the optical recording medium respectively used. The primary and secondary beams reflected from the optical recording medium are detected and evaluated in accordance with the known push-pull method in order to obtain the track error signal. In the process, considered by themselves in each case, both the primary beam and the secondary beams generate a push-pull signal which represents the track error of the respective signal with regard to the respectively scanned track. The desired track error signal can be generated by weighted combinations of the primary-beam and secondary-beam track error signals.
FIG. 7 illustrates a corresponding arrangement for carrying out the DPP method. The light emitted by a light source or a laser 1 passes through a collimator lens 2 and is then split into the primary beam (i.e. a 0th-order beam) and the two secondary beams (i.e. ±1st-order beams) by a diffraction grating 3. The primary beam, which reads the information to be scanned in a track of a corresponding recording medium 7, usually contains the majority (approximately 80-90%) of the light information. The two secondary beams each contain the remaining 5-10% of the total light intensity, it being assumed for the sake of simplicity that the light energy of the higher orders of diffraction of the diffraction grating 3 is zero. These three beams are focused onto the optical recording medium 7 via a polarizing beam splitter 4 and a quarter-wave plate 5 and also an objective lens 6, in order to read from and/or write to the said optical recording medium. The three beams reflected from the optical recording medium 7 are fed via the beam splitter 4 and a cylindrical lens 8 to a photodetector unit 9, which detects the three beams reflected from the optical recording medium 7. The three beams are only indicated symbolically here between cylindrical lens 8 and photodetector unit 9. Connected to the photodetector unit 9 is an evaluation unit 10, which evaluates the detected reflected primary and secondary beams for the purpose of generating a track error signal.
FIG. 8 illustrates an example of the imaging of the primary beam 14 and also of the two secondary beams 15, 16 onto a DVD-RAM as optical recording medium 7. In DVD-RAMs and also in some other types of optical recording media, information tracks are contained both in depressions, designated as “groove”, and in elevations, designated as “land”. In the context of the present patent application, the “groove” tracks are also abbreviated as G and the “land” tracks are abbreviated as L. Since the secondary beams 13 and 15 and the primary beam 14 are intended to be optically separable from one another, the positions of their imaging on the optical recording medium 7 and on the photodetector unit 9 are separate from one another. The diffraction grating 3 is incorporated in such a way that the imaging of the secondary beams impinges precisely on the centre of the secondary track or (in the case of storage media which are written to only in “groove” tracks) precisely on the region between two tracks beside the track read by the primary beam. If the optical recording medium 7 rotates, then one of the secondary beams is situated in front of, and the other secondary beam behind, the primary beam in the reading or writing direction. The evaluation unit 10 of the arrangement shown in FIG. 7 evaluates the light intensities reflected onto the photodetector 9 separately for each of the three beams.
Considered by themselves in each case, both the primary beam and the secondary beams generate, after evaluation by the evaluation unit, a push-pull signal which represents the track error of the respective beam with respect to the track. In the track image illustrated in FIG. 8, the two secondary beams illuminate the secondary tracks with respect to the read track; their push-pull track error component is therefore inverted with respect to that of the primary beam. Considered by themselves, the respective push-pull components thus contain the actual track error with respect to the respectively scanned track. Since the track position of the three beams can only change together, the three push-pull signals change equally.
If the objective lens 6 is then moved, for example in the event of a track jump, then the imaging of primary and secondary beams on the photodetector unit 9 also moves. This displacement of the imaging results in an offset voltage on the push-pull intermediate signals of the evaluation unit 10 which are provided individually for the three beams. The direction of this offset voltage is identical for all of the beams. The displacement of the objective lens 6 thus gives rise to an offset voltage which does not originate from an actual track error and is therefore an interference. The genuine track error component and the undesirable lens-movement-dependent component are added in the evaluated push-pull signal yielded by the respective detectors of the photodetector unit 9.
If the push-pull signals of the secondary beams are then added and this sum is subtracted from the push-pull signal of the primary beam, this undesirable lens-movement-dependent component is cancelled out given appropriate weighting between the primary and secondary beam components. However, since the push-pull components of the primary and secondary beams are inverted with respect to one another, they are added in the correct phase after application of the subtraction, with the result that, given correct setting of the weighting factor within the evaluation unit, all that remains is the actual track error. This will be explained in more detail below.
As has already been mentioned, the track error signal DPP is composed of the push-pull component CPP of the primary beam and the added push-pull components OPP of the secondary beams, the relationships specified in the following formulae (1)-(3) holding true.
                    CPP        =                              a            *                          sin              ⁡                              (                                  2                  ⁢                  π                  *                                      x                                          2                      ⁢                      p                                                                      )                                              +          kl                                    (        1        )                                OPP        =                              a            *                          (                                                sin                  ⁡                                      (                                          2                      ⁢                      π                      *                                                                        x                          +                                                      Δ                            ⁢                                                                                                                  ⁢                            x                                                                                                    2                          ⁢                          p                                                                                      )                                                  +                                  sin                  ⁡                                      (                                          2                      ⁢                      π                      *                                                                        x                          -                                                      Δ                            ⁢                                                                                                                  ⁢                            x                                                                                                    2                          ⁢                          p                                                                                      )                                                              )                                +                      2            ⁢            kl                                              (        2        )            DPP=CPP−K*OPP  (3)
In this case, K designates the weighting factor for the abovementioned weighted subtraction of the OPP signal from the CPP signal. The amplitudes a and k specified in the formulae are factors which depend on the geometry of the scanned tracks, the sensitivity of the photodetectors, etc. The scanning position of each beam relative to the track centre is designated by x, where Δx designates the distance between the two secondary beams and the primary beam. p designates the track spacing of the optical recording medium, in accordance with the definition in a DVD-RAM between “groove” and “land” centre of adjacent tracks, and l designates the movement of the objective lens 6 from its rest position. Since the three beams are mechanically coupled to one another, the variables x and l in formulae (1) and (2) are identical in each case.
For all the considerations below, it is assumed in a simplification that the intensities of the three scanning beams considered are identical when impinging on the photodetector unit 9. In practice, however, the intensity of the secondary beams is dependent on their track position, on the reflection of the track respectively scanned by the beams, and also on the properties of the optical grating, and is weaker than the intensity of the primary beam, with the result that the intensity of the secondary beams must correspondingly be scaled with respect to the primary beam intensity. This is ideally done by normalization.
In order to be able to compensate the lens-movement-dependent component, the following relationship must be satisfied:DPPl=CPPl−K*OPPl≅0  (4)where the index “l” designates the lens-movement-dependent component of the corresponding signal. Applying formulae (1) and (2), it follows that the lens-movement-dependent component l can be compensated if the following holds true:K=0.5  (5)
This weighting factor is independent of the orientation of the secondary beams with regard to the primary beam. It is usually attempted to make the track error amplitude a maximum by setting the distance Δx accordingly. With the value K=0.5 determined above, it is possible to express the above formula (3) for the track error component designated by the index “x” as follows:
                                                                        DPP                k                            =                                                a                  *                                      sin                    ⁡                                          (                                              2                        ⁢                        π                        *                                                  x                                                      2                            ⁢                            p                                                                                              )                                                                      -                                  0                  ⁢                                      ,                                    ⁢                  5                  ⁢                  a                  *                                      (                                                                  sin                        ⁡                                                  (                                                      2                            ⁢                            π                            *                                                                                          x                                +                                                                  Δ                                  ⁢                                                                                                                                          ⁢                                  x                                                                                                                            2                                ⁢                                p                                                                                                              )                                                                    +                                              sin                        ⁡                                                  (                                                      2                            ⁢                            π                            *                                                                                          x                                -                                                                  Δ                                  ⁢                                                                                                                                          ⁢                                  x                                                                                                                            2                                ⁢                                p                                                                                                              )                                                                                      )                                                                                                                          =                                                a                  *                  sin                  ⁢                                      (                                          2                      ⁢                      π                      *                                              x                                                  2                          ⁢                          p                                                                                      )                                                  -                                  0                  ⁢                                      ,                                    ⁢                  5                  ⁢                  a                  *                  2                  *                                      (                                                                  sin                        ⁡                                                  (                                                      π                            *                                                          x                              p                                                                                )                                                                    *                                              cos                        ⁡                                                  (                                                      π                            *                                                                                          Δ                                ⁢                                                                                                                                  ⁢                                x                                                            p                                                                                )                                                                                      )                                                                                                                          =                              a                *                                  sin                  ⁡                                      (                                          π                      *                                                                                                                                                        ⁢                          x                                                p                                                              )                                                  *                                  (                                      1                    -                                          cos                      ⁢                                              (                                                  π                          *                                                                                    Δ                              ⁢                                                                                                                          ⁢                              x                                                        p                                                                          )                                                                              )                                                                                        (        6        )            DPPx becomes a maximum when the following condition is satisfied:
                              cos          ⁡                      (                          π              *                                                Δ                  ⁢                                                                          ⁢                  x                                p                                      )                          =                  -          1                                    (        7        )            This is the case when the following holds true:Δx=(2n+1)*p where n=0, 1, 2, . . .   (8)
In accordance with the prior art, Δx=p is therefore chosen in the simplest case—as is shown in FIG. 8. FIG. 8 also illustrates the profile of the track error signals resulting from this beam arrangement in accordance with the prior art.
From the above-described properties of the DPP method according to the prior art, it is apparent that, owing to the position of the secondary beams, the phase shift between the push-pull signal CPP of the primary beam and the push-pull signals OPP1, OPP2 of the secondary beams is nominally 180°. This is advantageous, when the DPP method is considered as such, since, as a result of the difference formation, the track error components of the primary beam and of the secondary beams are added with the largest possible amplitude. The two secondary beam signals OPP1 and OPP2 are phase-shifted through 360° with respect to one another.
Owing to the phase shift of 180° between the primary beam signal CPP and the individual secondary beam signals OPP1, OPP2 and of 360° between the two secondary beam signals OPP1, OPP2, it is possible, with the aid of suitable comparators, to count the tracks of the optical recording medium 7 crossed by the objective lens 6 only without taking account of the direction of movement of the objective lens. In this case, as shown in FIG. 8, the comparators acquire the signals CPP, OPP1 and OPP2 and generate independently thereof, in this case at the zero crossing, signals KCPP, KOPP1 and KOPP2. By way of example, a so-called “track zero cross” signal TZC can be generated in a manner dependent on the signal KCPP. However, identification of the direction of movement of the objective lens or of the type of track respectively crossed is not possible in this way.
EP-A2-0 392 775 proposes forming the difference between the push-pull signals of the secondary beams and using the resultant difference signal for direction identification.